The invention relates to a current limiter for limiting current in case of a fault including two similar serially arranged inductivities or throttles and two similar controlled electric valves which are arranged in opposition to each other and parallel to the two inductivities.
A current limiter with controlled valves is known for example from DE 100 03 556. It comprises two serially arranged inductivities or throttles and two controlled electric valves which are arranged in opposition to each other in parallel with the two inductivities. In the known current limiter, the common potential point and the common potential point of the two inductivities are interconnected but not necessarily directly.
The current limiter is installed in the power current circuit of an apparatus. As explained in DE 100 03 556 in detail, it forms for the normal or nominal operation at most a negligible ohmic resistance. Only if a short circuit occurs in the apparatus to be served, that is, when a current threshold is exceeded, first one of the two valves and subsequently, the second valve are switched from a conductive state to a blocking state whereby an additional resistance is provided in the supply circuit of the apparatus for limiting the current.
The current limiter operates as follows: When the current circuit is switched on, after reaching the stationary stage, a circuit DC current I0 flows in each of the two circuits consisting of inductivity and bridged valve as a result of the effective component parameters. In the stationary nominal operation, the two inductivities present no resistance to alternating current since no time variable current is flowing. The two values are similar controlled valves which, when the power circuit is switched on, become actively conductive via the respective control electrode; with a possibly necessary voltage resistance, they consist each of a cascade of similar elementary valves.
The activation of the Fault Current Limiter, FCL, is indicated in case of a short circuit by the occurrence of an ohmic resistance, an inductivity or a combination of the two of a suitable level, whereby the short circuit current in the apparatus current circuit is limited. Up to nominal operation, the current limiter is not noticeable.
Upon occurrence of a fault/short circuit, when a predetermined fault current threshold is exceeded and upon first reaching the minimum duration for the restitution of the electrical strength for the presence of a voltage opposite to the passing direction at the respective valve, the valve becomes automatically non-conductive and remains non-conductive. This also occurs at the other then still conductive valve so that the two inductivities are finally arranged in a series circuit and act as an additional alternating current resistor in the electric circuit whereby the current or short circuit current remains within limits. For limiting the current in energy engineering heated reactors and superconductive throttles are used.
Particularly suitable for that purpose are superconductors since they have only a small voltage drop in normal operation and the transition to normal conductivity results in the elimination of a high electric resistance and causes the passing of magnetic fluxes. This can be utilized for a resistive or, respectively, inductive current limiter functions (Prof. Dr. Tech. P. Komarek, “High Voltage Application of the Superconductivity” (1995). In alternating current operation a superconductive wire is not without losses, but there are always two basis loss mechanisms in effect, that is, the eddy current losses in the matrix and the re-magnetization losses (hysteresis losses) in the hard superconductor. With DC current operation, the losses in a superconductive wire are practically zero.
In Boenig, H. J. and D. A. Paice, 1983, Fault Current Limiter using a Superconducting Coil, IEEE, Transactions on Magnetics, Vol. 19, No. 3, page 1051, May, a current limiter is disclosed whose superconductive throttle is switched on the DC current side by rectifiers. A voltage source is arranged at the DC current side in series with the throttle. The voltage source ensures that a current I0 flows through the throttle. The amount of the current I0 exceeds the amplitude of the AC current which flows through the limiter. As long as there is no fault, it is substantially lower than the short circuit current. The bridge then also permits the load current to pass. Herein, the voltage drop across the limiter equals the voltage drop across the valves in the two shoulders of the bridge.
FR-No. 1 337 971 discloses an electric control circuit, which comprises a load which is connected to an electric energy source via a protective circuit. The protective circuit limits the current changes in order to mitigate a sudden current change in the load. The protective circuit comprises two inductivities which are arranged in series and which are bridged by two uncontrolled valves or diodes arranged in series. The diodes have opposite low resistance directions. The common points of the diodes and the inductivities are directly interconnected. The uncontrolled valves block only currents which are greater than the operating current of the protective arrangement which corresponds to the operating current of the inductivities, and conduct the high current components via the inductivities almost without resistance (diode resistance in low resistance direction). The current flowing in the inductivities cannot follow the supply current which drops again after the current maximum since it is short circuited by the diodes and, in the process, remains essentially at the given value.
The consideration of each respective next half wave is divided into a consideration of currents below the operating current and those above. Smaller currents flow via the conductive diode whereas the higher currents flow via the conductive diode whereas the higher currents flow via the inductivities and increase the current flowing this way already. This process is repeated with each half wave for the respective effective part of the circuit. During a short circuit, this results in an essential increase of the current in the inductivities and in the outer current circuits during the minimum time interval which is needed for switching the arrangement off (100-150 msec.) and particularly during the time interval (1-2 secs) which is needed for securing the selective operation of the protective devices.
By the use of active semi-controlled valves such as thyristors in a bridge circuit, a more effective limitation of the short circuit current is achieved. As a result, a power output control is obtained which, during a fault, controls the current flowing through the coil down to zero (Boenig, H. J. and D. A. Paice, 1983, Fault Current Circuit Limiter using a superconducting Coil, IEEE Transactions on Magnetics, Vol. 19, No. 3, page 1051, May; FR-Nr. 1 337 971). However, in this case, a special apparatus is needed which identifies a short circuit and changes the control algorithm for the thyristor in the bridge, that is, in this case, it is not a current limiter which operates safely with passive triggering. In this case, the current limiter is therefore not an element that can be passively triggered. It rather requires a special control unit for the thyristor. This greatly reduces the reliability of the current limiter.
In case of a fault, when the amplitude of the alternating current exceeds the value I0 in the throttle, the short circuit current is limited by the inductivity of the throttle. The limit for the short circuit current is determined by the maximum energy which is stored in the throttle during the transient process:Wmax=L·Imax2/2
Optimization of the current limiter means minimizing the energy (Wmax) stored in the throttle.
In a current limiter as described in DE 100 03 556, the valves of the current limiter trigger themselves spontaneously during a fault, when a predetermined high current above nominal current is exceeded, that is, they act passively. They switch from a conductive state to a blocking state and remain in the blocking state. The two inductivities L act during the fault occurrence as additional alternating current resistor 2ωL in the current circuit whereby, with a corresponding dimensioning thereof, the short circuit current is limited for compatibility with and protection of the apparatus.
The current limiter described in DE 100 03 556 is provided for the rare fault occurrence of a short circuit in an apparatus in which it is installed. The fault remains under control as long as the two valves are operational. The protective function of the two valves is very important for the respective apparatus. The very rare fault occurrence of failure or breakdown of a valve or of both valves would be catastrophic since there would no longer be an effective current limit during a short circuit. The attrition of the electric energy present in the electric circuit would find no resistance, would occur instantly and result in a destructive energy change in a very short time at least until finally supervisory protection systems are activated and effectively put in place.
It is therefore the object of the present invention to provide the known current limiter for limiting an apparatus current during a fault occurrence with a passively effective, spontaneously reacting protective arrangement which limits a short circuit current during a failure or breakdown of one or both valves right from the beginning of such a failure at least for such a time until the apparatus or the network can be fully shutdown by way of the available protection equipment so that no additional damage will occur.